1. Technical Field
The present invention relates to a technique of controlling a variety of electro-optical elements such as organic light emitting diode (hereinafter, referred to as “OLED”) elements.
2. Related Art
An electro-optical device having such a kind of electro-optical elements includes a plurality of electro-optical elements arranged in a planar shape corresponding to a plurality of data lines and a plurality of current output circuits generating data signals on the basis of digital data (hereinafter, referred to as “gray-scale data”) defining gray scales of the electro-optical elements and outputting the data signals to the data lines. Each current output circuit has a function as a digital-to-analog converter including a plurality of transistors (hereinafter, referred to as “current supply transistors”) serving as a current source and generates the data signals by adding current flowing in one current supply transistor selected in accordance with the gray-scale data among the current supply transistors.
Errors may occur in characteristics (specifically, threshold voltage) of the plurality of current supply transistors included in each current output circuit due to reasons in manufacture. In this way, when deviation occurs in characteristics of the current supply transistors, the current supply transistors cannot generate the data signals having a predetermined current value corresponding to the gray-scale data. As a result, there is a problem that display quality is deteriorated.
In order to solve the problem, a configuration that a circuit (hereinafter, referred to as a “compensation circuit”) compensating for the deviation in characteristics of the current supply transistors is disposed in each current output circuit is disclosed in JP-A-2004-88158 (see paragraph 0053 and FIG. 3). Each compensation circuit includes a transistor (hereinafter, referred to as “compensation transistor”) of which the drain terminal and the gate terminal are connected to each other and a capacitor holding the voltage of the gate terminal thereof. The compensation transistor has substantially the same characteristic as the respective current supply transistors. When the voltage (hereinafter, referred to as “reference voltage”) of the gate terminal of the compensation transistor which has been temporarily turned on is supplied to the gate terminal of the respective current supply transistors, the errors of the current supply transistors are compensated for.
However, when the reference voltage is once varied due to noises or the like, the voltage of the gate terminal of the compensation transistor is kept with the level after the variation. Accordingly, there is a problem in that the reference voltage with a predetermined level cannot be supplied to the gate terminals of the current supply transistors and thus it is difficult to control the data signals in a desired current value.